1. Field of the Invention
The present invention relates in general to superconducting cables, that is to say cables intended for transporting electric current in so-called superconductivity conditions, namely in conditions of almost zero electrical resistance in the regime of transportation of direct current.
More in particular, the invention relates to a method for reducing alternating current (AC) losses in a superconducting conductor element subjected to an external magnetic field and through which a current flows. Furthermore, the invention relates to a superconducting conductor element. Furthermore, the invention relates to a superconducting phase element. Furthermore, the invention relates to a warm dielectric superconducting cable.
In the following description and in the following claims, the expression “superconducting conductor element” is meant to indicate the electrically active portion of a superconducting cable, comprising a support and at least one layer of superconducting material, intended for transporting the phase electric current or, in the case of a three-phase current, each phase current.
The expression “superconducting phase element” is meant to indicate each component of a superconducting cable associated with the or each phase, comprising a superconducting conductor element and the respective electrical and thermal insulations.
The expression “warm dielectric” (WD) is meant to indicate a structure of coaxial layers of a superconducting phase element comprising, essentially, a tubular axial support and at least one layer comprising superconducting material (that is to say a superconducting conductor element), a cryostat and a dielectric, wherein a fluid, typically liquid nitrogen, flows within the support element for cooling the superconducting material below its critical temperature.
The expression “superconducting material” is meant to indicate a material, as for example special niobium-titanium alloys, or ceramics based on mixed oxides of copper, barium and yttrium (YBCO) or of gadolinium, samarium or other rare earth (REBCO), or of bismuth, lead, strontium, calcium, copper, thallium and mercury (BSCCO), one of which phases has, below a certain temperature defined as critical temperature (TC), an almost zero resistivity, in the regime of transportation of direct current.
The superconducting material is commonly used in the form of tapes wound around a substantially tubular support element. Tapes containing a film of superconducting material (YBCO or REBCO) supported by a steel tape, optionally coated with one or more layers of oxide, and tapes wherein filaments of superconducting material (BSCCO) are embedded in a metallic matrix are well-known. The present description and the attached claims refer to both types of tape with the expression “tapes comprising superconducting material”.
The expression “transport current” is meant to indicate a current flowing in a tape comprising superconducting material, in a superconducting conductor element, in a superconducting phase element, or in a superconducting cable, according to the circumstances.
2. Description of the Related Art
In the field of superconducting cables, a particularly important problem is that of minimizing the AC losses.
The losses in a superconducting material are essentially of a hysteretic nature, due to the intrinsic dissipation of the superconducting material caused by the penetration of a magnetic field within the superconducting material itself.
The losses of a hysteretic nature are added to the losses due to eddy currents, that is to say ohmic losses of the currents which are induced, by variable magnetic fields, in the metallic areas of the superconducting cable in general.
A first magnetic field causing losses is that generated by the transport current itself. This magnetic field is commonly referred to as “self-field”.
External magnetic fields of particular interest to practical applications of superconducting cables are those due to the presence of the other phases in a three-phase or polyphase cable.
The expression “polyphase cable” is meant to indicate a cable wherein the current of each phase, in a single phase or three-phase current system, is distributed among various superconducting phase elements.
Other external magnetic fields of particular applicative interest are due to the presence of a generator, an engine or a current limiter near to a superconducting cable.
In order to eliminate or substantially reduce the losses due to external magnetic fields, cold dielectric (CD) superconducting cables are well known. In each superconducting phase element, these kinds of superconducting cables have at least one return layer comprising superconducting material, coaxial to that intended for transporting the current, and shielding the latter from external magnetic fields. Such cables however have rather high initial costs due to the practically double quantity of superconducting material used compared to warm dielectric cables.
In warm dielectric superconducting cables, with which the present invention is especially concerned, however, the superconducting material for transporting the current is not shielded.
The effect of magnetic coupling between the phases of a three-phase warm dielectric superconducting cable on the losses in a superconducting conductor element through which a transport current flows has been studied in the paper of J. O. Willis et al., “Single and Multi-Phase ac Losses in HTS Prototype Power Transmission Conductors”, IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, Volume 9, June 1999, page 778.
The results, indicated therein, of the measurements carried out indicate that the losses in three-phase configuration, that is to say the total losses in the presence of a transport current and of the magnetic field generated by the current of the other two phases, flowing in resistive conductors, are greater than the sum of the losses due to the sole transport current (single-phase configuration) and the losses due to the sole current in the conductors of the other phases (two-phase configuration), or, briefly, that the AC losses have a non-linear nature.
According to this paper, furthermore, the losses in the single-phase configuration decrease with increasing critical current of the superconducting conductor element.
The expression “critical current (density)” is meant to indicate the current (density) that generates at the ends of a superconducting conductor element an electric field equal to 1 mV/cm.
In the article by K. H. Müller, “Self-field hysteresis loss in periodically arranged superconducting tapes”, Phisica C 289, pages 123–130, 1997, an analytical calculation of the magnetic field distribution, of the current distribution and of the self-field losses in two configurations of superconducting strips through which a transport current flows is presented.
The configurations studied are presented as an idealized model of the arrangement of tapes comprising superconducting material in superconducting conductor elements of superconducting cables.
More in particular, the z-stack configuration is studied, i.e. an infinite series of strips placed with the wide faces adjacent and spaced out and the x-array configuration, i.e. an infinite series of strips placed with the narrow faces adjacent and spaced out.
Such a paper concludes that the self-field AC losses per strip decrease with decreasing gap between the strips and are minimal with a zero gap.
In this regard, the Applicant observes that a zero gap is an ideal condition, difficult to put into practice. In fact, for technological reasons, in the machine production of superconducting conductor elements it is exceedingly difficult to wind the superconducting tapes with continuity. Furthermore, in the case of metallic matrix/multifilament tapes, as described in further detail below in the present description, even by reducing to zero the distance between the tapes, a gap still remains (in the order of about 0.6 mm) between the superconducting material of adjacent tapes, due to the presence of an edge area of the metallic matrix which is free of superconducting filaments.